The production of stainless steel seamless tube will primarily rely on cold drawing and/or cold rolling (pilgering) machines to reduce the tube to desirable tube diameter and/or wall thickness. The mechanically worked tube will thus be required to go through annealing process, a critical production step to reduce the hardness of a material to facilitate the progress of subsequent manufacturing operations. Traditional production of stainless steel seamless tube generally use open-hearth fossil-fuel-fired furnace. As a result of open-hearth annealing, a black oxidize layer will be formed on the tube surface due to exposure to atmospheric oxygen. The cleaning of this blacken oxidized layer is then accomplished by soaking the tube in a pickling tank over long period of time. Thus, the traditional production adopting open-hearth fossil-fuel-fired furnace is time consuming and environmentally unsound.
Alternatively the tube annealing, after the drawing and/or rolling steps, can be achieved in a bright annealing furnace by burning hydrogen to the annealing temperature in a sealed chamber filled with hydrogen. Since hydrogen is an inert gas, a black oxidize layer will not be formed on the tube surface. The resultant uniform and smooth tube surface has thus earned the hydrogen-fired furnace the name of ‘bright annealing furnace’.
Cold rolling is employed to reduce tube outer diameter and/or wall thickness in the production of bight annealing stainless steel seamless tube. The application of rolling machine requires the use of high viscosity heavy lubricants and this leaves behind remnant lubricant on both outer and interior tube surfaces. Cleaning the exposed outer tube surface will not present any problems. However, due to lack of access into the tube interior, particularly for tubes with small bore, thorough cleaning of tube interior surface is a major challenge. Conventional methods employed to clean lubricant residue from interior surface of the tube is the mechanical scrubbing scheme. Tightly squeezed sponge or cotton balls, soaked with chemical solvent such as acetone and inserted in one end of the tube, shot through the tube bore by a high-speed gas gun.
Further, ultrasonic vibration is also employed to remove blob of lubricant residue from the interior of tube surface. For ultrasonic cleaning, tubes are placed in a tank and soaked in warm and soaping water and are subjected to high frequency vibrations for hours to shake away remnant lubricant from the tube surface. For better cleaning results, tubes were subjected to ultrasonic vibration first and then followed by mechanical scrubbing. However, for tubes with very small outer diameter, these conventional methods still fail to eliminate the very thin film of lubricant remnant present on the interior surface of the tube. Within this very thin layer, known as boundary layer in viscous fluid dynamics, the remnant lubricant will firmly stuck to the adjacent tube surface. The thickness of this layer, depending on the lubricant viscosity and rolling pressure or tube reduction ratio, varies from a few micrometers to less than one micrometer. This thin film will induce slight but uniform discoloration on the interior surface of the tube. In addition, this thin film contains hydrocarbons which may prove to be harmful for certain critical tube applications.
In the light of the above discussion, there appears to be a need a new and novel approach to eliminate the presence of this thin film of remnant lubricant from the interior surface of the tube produced by bright annealing furnace that will overcome the existing shortcomings of the conventional cleaning methods, employed alone or in combination.